Analysis of c-ets gene expression during spermatogenesis, thymus development and during compensatory growth of liver indicates that (i) ets-2 gene expression is linked to cell proliferation and occurs before DNA synthesis, (ii) ets-2 gene expression may be regulated during the course of development, (iii) both ets-1 and ets-2 genes are differentially regulated, and (iv) ets genes may belong to the nuclear family of oncogenes. In mice, the ets-2 gene is transcribed as a major mRNA species of 4.2 kb and expressed in most of the tissues examined. The ets-1 gene is transcribed as multiple mRNA species sized 7.5 kb, 2.4 kb and 1.7 kb. The product of ets-2 appears to be preferentially expressed as a protein of 56 Kd in size, and has been identified as a putative ets-2 gene protein which is expressed at much higher levels in the thymus. The role of ets gene products in cell proliferation and differentiation is under investigation. Human myc genes containing entire coding sequences or only the second and third exons have been expressed under the control of the metallothionein promoter, using the bovine papillomavirus (BPV) vector system. Permanent cell lines expressing human myc proteins have been established. Analysis of human myc gene products in these cell lines indicates that (i) myc gene products enhance BPV-induced transformation, (ii) 62-64-Kd human myc protein is made either when all three exons are present or only the second and third exons are present, (iii) human myc protein expressed in mouse cells is mainly compartmentalized in the nucleus, (iv) human myc protein is inducible with heavy metal ions, and (v) though the myc gene is present on an episome in the cell, it appears to be subject to a similar regulatory control mechanism(s) like those controlling the endogenous c-myc gene.